There's plenty of cool stuff to do with GPS, but this may be one of the coolest things, at least if you like to spend your weekends with your butt firmly planted on a bicycle seat: mapping out bicycling routes and rides. I'm a semi-frequent road biker. Recently bought a pair of the fancy shoes. I'm not serious enough yet to ride with a GPS monitor strapped to my arm, but if I was, I'd be all over the MotionBased Web site. It's a subscription service, based in Sausalito, Calif. (near Mt. Tam, where mountain biking was invented) that allows you to map and analyze your biking activities. (also hiking, canoeing, running, etc.)
Using a computer and cable you can upload all sorts of info about your rides, including time, distance, elevation, and heart rate, and weather. Once the info is in the system, you can do all sorts of comparing and analyzing your performance on different routes. You can also link your MotionBased data with Google Earth to superimpose your routes on Google Earth satellite maps.
There's some really interesting community stuff, as well. You can share your routes and performance data with others. One amazing feature: "Dot racing." You feed in your data about a recent ride on a particular route, others do the same, and you can watch dots representing the different riders "race" around the route laid out on a map.
MotionBased has a blog.
Here's a site that links to all sorts of GPS/recreation sites.
Also check out my biking friend and BW colleague Steve Baker's recent blog item asking about biker route sharing, "Wanted: Global Bike Maps."